1. Field of the Invention
The present invention relates to a halftone dot producing apparatus for producing a plurality of halftone dot image data representative of a plurality of monochromatic images in which a color image is separated, and a halftone dot producing program storage medium storing a program which causes a computer system to operate as the halftone dot producing apparatus.
2. Description of the Related Art
Conventionally, a color image print is produced by reading in a color image by a color scanner and editing the color image thus read (page make-up); reading in multi-tone level image data of color plates (e.g., yellow (Y), magenta (M), cyan (C) and black (K)) that are representative of the image completed by the page make-up; producing halftone dot image data of YMCK color plates according to the multi-tone level image data of those color plates by using a halftone dot producing apparatus; producing plate making films for the color plates carrying the halftone dot images of YMCK color plates according to the halftone dot image data of the color plates; printing of printing plates for the color plates by using the plate making films; and printing, on a predetermined medium, a color image in which the halftone dot images of the YMCK color plates are superposed on one another, using those printing plates with a predetermined printing machine.
Generally, as described above, the printing of the color image is on a large scale. Thus, an operator, who takes part in producing the color image print, uses a proofer to produce a simple image (proof) of the color image before the final printing, and verifies the quality of the final printed color image by referring to the proof image outputted from a printer in accordance with the proofer.
The halftone dot images of the YMCK color plates, which constitute the color images, are composed of screens of halftone dots arranged with slopes of angles 0°, 15°, 45° and 75°, for example, respectively. In those halftone dot images, a gradation of the image is represented by an area modulation for the individual halftone dot. An arrangement of the halftone dots in the halftone dot images is defined by the number of lines representative of the density of the lines consisting of columns of halftone dots other than the above-mentioned angles. The color image, in which the halftone dot images are superposed on one another, is associated with a Rosette pattern that is peculiar to the printed image because of the superimposition of the MCK plates (the Y plate is excluded because of its light color).
The conditions that produce a Rosette pattern have previously been analyzed. For example, as explained in Japanese Patent Application Laid Open Gazette Hei. 2-58176, a Rosette pattern is formed by setting up the number of lines of the halftone and the angles in such a manner that a row of the halftone dots of the screen is grasped as a wave and a secondary moiré is prevented from occurring because of the overlapping of a primary moiré, produced by the overlapping of a screen of an angle 15° and a screen of an angle 75°, and a screen of an angle 45°. That is, the primary moiré and a wave number component of the screen of the angle 45° are coincident with one another.
A combination of the angles of the halftone dots used in a print, is generally 15°, 45° and 75°. It is important that a relative angle between those angles is at intervals of 30 degrees, and it is noted that the combination is not restricted to the angles as mentioned above. For example, it is known that a Rosette pattern also occurs in a combination of 22.5°, 52.5° and 82.5°. Generally, the halftone dots are arranged as a tetragonal lattice, and thus it is arranged in angles added with ±90° or 180°. Accordingly, it happens that the angles of the halftone dots are referred to in angles added with ±90° or 180°.
It is well known, as shown at pages 56 to 57, “Post Script Screening” (Inter Press) written by Peter Fink, that there are two sorts of Rosette patterns of “clear center” and “dot center”.
FIG. 25 is an illustration showing Rosette pattern of the clear center. FIG. 26 is an illustration showing Rosette pattern of the dot center.
As shown in FIG. 25, Rosette pattern of the clear center has a center portion a1 which is cleared, and has a pattern of halftone dots expanding as a circle in the vicinity of the center portion a1. And as shown in FIG. 26, Rosette pattern of the dot center has a center portion a2 which is dotted, and has a pattern of halftone dots expanding as a circle in the vicinity of the center portion a2.
A clear-centered Rosette pattern and dot-centered Rosette pattern may be changed therebetween by shifting, of three screens constituting those patterns, a superimposing position of one screen with respect to other screens, for example. In accordance with an image, a selection between a clear-centered Rosette pattern and a dot-centered Rosette pattern can be made with respect to a set of halftone dot image data of YMCK color plates on RIP (Raster Image Processor) for developing information of characters and images into a bit map, which is connected to a film setter that outputs the plate making film. The difference between the Rosette patterns is caused by a difference in the superpositioning of halftone dots of the screens by the deviation of a superimposing position of the above-mentioned three screens. Hereinafter, positions of the above-mentioned three screens constituting a clear-centered Rosette pattern, for example, are defined as reference positions, and the positions of the three screens with respect to the reference positions are referred to as phases of the halftone dots on the screens, respectively.
However, in various steps of the conventional color image printing method, as described above, a register discrepancy can occur in which positions of the color plates are deviated. For example, even if halftone dot image data is related to a Rosette pattern representative of the clear-center or the dot-center, the expected clear-centered Rosette pattern or the expected dot-centered Rosette pattern does not appear.
A Rosette pattern is preferable in the sense that the occurrence of the secondary moiré is suppressed. However, if the Rosette pattern is clearly generated as shown in FIGS. 25 and 26, the Rosette pattern is visible to the naked eye and thus, the Rosette pattern is not preferable on the basis of image quality. Hereinafter, a Rosette pattern, which is not preferable on the basis of image quality, such as the clear-centered Rosette pattern or dot-centered Rosette pattern described above, is referred to as a Rosette moiré. In various steps of the conventional color image printing, as described above, a register discrepancy is generated. Thus, in the actual print, a Rosette moiré will occur if a phase between the color plates due to the register discrepancy is near a phase implementing a clear-centered Rosette pattern and a dot-centered Rosette pattern described above.
As a method of suppressing the Rosette moiré, Japanese Patent Application Laid Open Gazette Hei. 2-134635 proposes a method in which a phase of halftone dot of one color plate is shifted to an intermediate phase between a phase implementing the clear-centered Rosette pattern and a phase implementing the dot-centered Rosette pattern. This intermediate phase is far from the phases implementing the clear-centered Rosette pattern and the dot-centered Rosette pattern. Thus, this method makes it possible to suppress the occurrence of Rosette moiré in cases where the register discrepancy of the color plate in the various steps of conventional color image printing is small.
Recently, advances in the digitalization of the printing process, and digitization of the page make-up of page data has improved register accuracy so that the registration of the color plates at the time of the page make-up has improved. Further, in CTP (Computer To Plate) printing, where digital data is directly written onto a printing plate and in CTC (Computer To Cylinder) printing, where digital data is directly written onto a printing plate on a cylinder of a printing machine, the registering of the position of the color plates at the time of the page make-up is implemented with greater accuracy. For this reason, if no phase control of halftone dots of the color plates is performed, there is a tendency that a Rosette moiré, which was not obvious until now because of register discrepancy, appears on the printed image and is visible to the naked eye. The occurrence of the Rosette moiré can be suppressed in accordance with the above described method in which a phase of halftone dot of one color plate is shifted to be an intermediate phase between a phase implementing the clear-centered Rosette pattern and a phase implementing the dot-centered Rosette pattern, and the intermediate phase is far from any of the phases implementing the clear-centered Rosette pattern and the dot-centered Rosette pattern. However, in a situation where great accuracy in registering has been implemented, as described above, it is desirable that the phase is freely changed so that the Rosette pattern is altered in accordance with the user's request, without the need to fix the phase onto the above-mentioned intermediate phase in order to avoid the Rosette moiré.
In the processes of printing using a film setter according to the prior art, it often happens that a register discrepancy occurs when producing the plate making film and the printing of the printing plate, and thus, in some cases, the Rosette moiré is not generated on the final print. However, a proofer generally plots position with great accuracy, and particularly, if a phase control for halftone dots of the color plates is not performed, a Rosette moiré appears on the outputted proof image. Although the proof image is produced in approximation of the final image print, generation of such a Rosette moiré will bring about the possibility that the proof image is different in impression from the final image print. For this reason, it is desirable to enhance a reproducibility of the proof image with respect to an image print by means of controlling a phase of halftone dots in at least one color plate of the color plates constituting a proof image in accordance with a degree of a register discrepancy of the image print.